With the advancement of ubiquitous network society, there is a large demand for mobile devices such as cellular phones, notebook personal computers, and digital still cameras. As the power source for mobile devices, it is desired to put fuel cells into practical use as early as possible since fuel cells do not need charging and permit continuous use of such devices if they are resupplied with fuel.
Among fuel cells, direct oxidation fuel cells are receiving attention and their research and development is actively conducted. Direct oxidation fuel cells generate power by directly supplying an organic fuel, such as methanol or dimethyl ether, to an anode and oxidizing it without reforming it into hydrogen. Organic fuels have high theoretical energy densities and are easy to store. In addition, the use of an organic fuel enables simplification of fuel cell systems.
Direct oxidation fuel cells have a cell structure in which an electrolyte membrane-electrode assembly (MEA) is sandwiched between separators. The MEA typically includes an electrolyte membrane sandwiched between an anode and a cathode, and each of the anode and the cathode includes a catalyst layer and a diffusion layer. Such a direct oxidation fuel cell generates power by supplying a fuel and water to the anode and supplying an oxidant to the cathode.
For example, the electrode reactions of a direct methanol fuel cell (DMFC), which uses methanol as the fuel, are as follows.Anode: CH3OH+H2O→CO2+6H++6e−Cathode: 3/2O2+6H++6e−→3H2O
On the anode, methanol reacts with water to produce carbon dioxide, protons, and electrons. The protons migrate to the cathode through the electrolyte membrane and the electrons migrate to the cathode through an external circuit. On the cathode, these protons and electrons combine with oxygen to produce water.
However, putting direct oxidation fuel cells into practical use has some problems.
One of the problems is unevenness of fuel concentration in the fuel flow channel of a separator. A fuel supplied to the anode is gradually consumed by power generation and fuel crossover (phenomenon in which a fuel supplied to the anode migrates to the cathode through the electrolyte membrane without reacting) while passing through the fuel flow channel. Thus, the fuel concentration on the fuel outlet side becomes lower than that on the fuel inlet side, thereby causing a gradient of fuel concentration in the anode. Since the speed of electrode reaction is proportional to the fuel concentration, an imbalance of electrode reaction tends to occur between the fuel inlet side and the fuel outlet side, thereby resulting in a decrease in the durability of the MEA.
Further, fuel crossover lowers the fuel utilization rate or cathode potential, thereby resulting in degradation of power generating characteristics.
To suppress unevenness of fuel concentration in the fuel flow channel, there have been many proposals to divide the fuel flow channel in order to reduce the length of the flow channel.
For example, Japanese Laid-Open Patent Publication No. 2006-156398 (Document 1) discloses a separator having a plurality of flow channels. These flow channels have substantially the same length, and each of the flow channels is divided into a plurality of groups. Such structure reduces the gradient of fuel concentration in the respective flow channels and decreases the pressure loss in the respective flow channels.
Japanese Laid-Open Patent Publication No. 2005-108688 (Document 2) proposes a separator having two fuel flow channels that are symmetric with respect to a line. The separator has one fuel inlet and two fuel outlets. The fuel inlet is disposed in the central part of a predetermined side of the separator, and the two fuel outlets are arranged on the side opposite to the side with the fuel inlet such that they are symmetric with respect to a predetermined axis. Such structure can suppress the pressure loss in the fuel flow channels and make the fuel concentration distribution and temperature distribution uniform.
Also, Japanese Laid-Open Patent Publication No. Hei 8-45520 (Document 3), which is not directed to a direct oxidation fuel cell, discloses a gas flow channel that spirally winds inward and makes a reverse turn in the center thereof. Such structure minimizes the change in the flow direction of gas and can reduce the pressure loss.
Further, to reduce fuel crossover, there have been many proposals to improve anode structure.
For example, Japanese Laid-Open Patent Publication No. 2002-110191 (Document 4) discloses that the methanol permeation coefficient of the anode diffusion layer is made greater more downstream of the fuel flow channel. Since methanol crossover in the first half of the fuel flow channel and methanol shortage in the latter half of the fuel flow channel are suppressed, fuel can be uniformly supplied to the anode. In Document 4, the anode diffusion layer comprises a carbon paper substrate and a mixed layer formed on the substrate. The mixed layer includes an electronically conductive porous material such as carbon black and a water-repellent binding material such as polytetrafluoroethylene. The methanol permeation coefficient is adjusted by, along the flow direction of fuel, (i) reducing the thickness of the mixed layer, (ii) decreasing the weight ratio of the water-repellent binding material, (iii) lowering the water repellency of the electronically conductive porous material, and/or (iv) increasing at least one of the porosity and the pore size of the electronically conductive porous material.
However, according to the above-mentioned conventional art, it is not possible to reduce the gradient of fuel concentration in the power generation section and hence ensure uniform supply of fuel. It is therefore difficult to provide a direct oxidation fuel cell which does not suffer from a decrease in fuel utilization rate and has excellent power generating characteristics.
For example, as in the techniques disclosed in Documents 1 and 2, even if the fuel flow channel is divided to reduce the length of the flow channel, it is not possible to drastically solve the problems of excessive supply of fuel upstream of the fuel flow channel and fuel shortage downstream thereof.
Since fuel permeates and diffuses through the diffusion layer, the fuel is not necessarily supplied to the anode along the flow direction of fuel in the fuel flow channel. Hence, even if the technique disclosed in Document 3 is used, it is difficult to reduce the gradient of fuel concentration.
With respect to the technique disclosed in Document 4, sufficient consideration is not given to the influence of, for example, methanol concentration and operating temperature on the methanol permeation coefficient of the anode diffusion layer. Therefore, for example, when high concentration methanol is used as the fuel or when the operating temperature is set high, methanol crossover increases, thereby resulting in a significant increase in the gradient of methanol concentration in the anode.
In view of the problems as described above, it is an object of the invention to provide a fuel cell which does not suffer from a decrease in fuel utilization rate and has excellent power generating characteristics.